| BREEDING,CULTIVARS,ROOTSTOCKS, AND GERMPLASM RESOURCES

HORTSCIENCE 53(6):777–781. 2018. https://doi.org/10.21273/HORTSCI12748-18 racesA,B,C,andD.B2Carosefrom crosses of PI 195342, ‘Bridgeton’ and US 1068, a sibling of ‘Bridgeton’ (Thomas and Inheritance of Resistance in Fisher, 1980). Phytophthora phaseoli The lima bean improvement program to , the Causal conducted by USDA-ARS, Beltsville, be- tween 1949 and 1989, resulted in significant Agent of Downy Mildew of Lima Bean improvement in quality, yield, and most importantly, resistance to the four prevalent 1 Luisa Santamaria races of P. phaseoli (Evans et al., 2007; Kee Department of Botany and Pathology, North Willamette Research & et al., 1997). From this program, more than Extension Center, Oregon State University, Aurora, OR 97002 30 cultivars and germplasm releases were made available to plant breeders, seed com- Emmalea G. Ernest, Nancy F. Gregory, and Thomas A. Evans panies, and producers (Stavely, 1991). Department of Plant and Soil Sciences, University of Delaware, Newark, DE For 20 years, no new races of P. phaseoli 19716 were detected in the Mid-Atlantic region (MAR). However, in 1995, a new race of P. Additional index words. lunatus, oomycete, screening for disease resistance, phaseoli, designated race E, was detected in germplasm collection, single dominant gene Delaware and became more widely distrib- Abstract Phytophthora phaseoli uted than race D (Evans et al., 2002). In 2000, . The oomycete is one of the most threatening pathogens of an epidemic incited by race E resulted in an lima bean (Phaseolus lunatus) in the humid Mid-Atlantic United States. In the last 60 P. phaseoli estimated loss of 40% of Delaware lima bean years, has evolved to overcome genetic resistance in the host and several production, equal to a farm gate value loss physiological races have been identified during the last 6 decades. Six physiological races of $3,000,000 (Scuse and Feurer 2003). A, B, C, D, E, and F have been identified over the years. Only race F has been detected in Then, race F of P. phaseoli was first detected the field over the past decade. Identifying and characterizing sources of resistance to this in 2000 (Evans et al., 2005) and was the most pathogen and determining the nature of resistance were the main objectives. Eight prevalent race by 2004. Because of the commercial cultivars and 35 germplasm accessions of P. lunatus were evaluated for their emergence of the E and F races, it is impor- reaction to races E and F. Four commercial cultivars and four accessions with resistance to race E, and two cultivars and four accessions with resistance to race F were identified. tant to evaluate the reaction of the cultivars None of the germplasm evaluated were resistant to both races. Five populations of F2 most commonly grown in Delaware to these and a recombinant inbred line (RIL) population were produced and inoculated to new races and identify possible sources of investigate the inheritance of resistance to races E and F. Resistance to races E and F was resistance. There are currently no cultivars determined to be conferred by single, independent, dominant genes. with resistance to both races E and F of P. phaseoli, resistance to race F is needed in horticulturally acceptable germplasm for use Downy mildew, incited by P. phaseoli races A, B, C, D, E, and F have been reported in the humid MAR. Thaxt., became a major threat to lima bean by Evans et al. (2007). In 2004, a lima bean breeding program (P. lunatus L.) production in the humid The USDA downy mildew resistance was re-initiated at the University of Delaware eastern United States in the 1940s. The U.S. breeding program began in 1948 and in to develop new cultivars for the MAR, in- Department of Agriculture (USDA) began 1958, released ‘Thaxter’, its first cultivar cluding cultivars with resistance to races E breeding lima bean for resistance to downy with downy mildew resistance (Wester and and F of P. phaseoli. This study was un- mildew in 1948 in collaboration with the Cetas, 1959). The resistance in ‘Thaxter’ was dertaken to support the disease resistance Long Island Vegetable Research Farm at conferred by a single dominant gene derived breeding goals of the program. The first Riverhead, NY (Wester and Cetas, 1959). from the P. lunatus landrace PI 164155. A objective of was to evaluate the reactions of At that time, no races or variants of the new race of P. phaseoli, which overcame the commercial cultivars to races E and F of P. pathogen were known. A physiological race, resistance in ‘Thaxter’, was detected in 1958 phaseoli in the field. The second objective hereafter referred to as race, is a subdivision and was designated race B (Wester and Cetas, was to identify sources of resistance to those of a pathogen species, particularly fungi, 1959). The landrace PI 189403 was found to races from diverse germplasm, including distinguished from other members of the be resistant to races A and B (Wester and cultivars, germplasm releases, and landraces. species by specialization for pathogenicity Cetas, 1959) and was used as the resistant The third objective was to determine the to different cultivars of a host (Kirk et al., parent in the development of ‘Dover’, which pattern of inheritance of resistance to these 2001). The use of differential cultivars has was released by the USDA in 1970 (Stavely, two races to facilitate incorporation of re- been the principal method to distinguish 1991). A new race of downy mildew, desig- sistance genes into commercially acceptable emerging races of P. phaseoli. Differential nated C, which overcame the resistance in small, flat-seeded (baby), and large, plump- cultivars used to distinguish physiological Dover, was identified in 1969 when the seeded (Fordhook), lima bean cultivars for cultivar was still in trial (Wester, 1970). production in the MAR. The PI 195342 was found to be resistant to Received for publication 5 Jan. 2018. Accepted for the new race C as well as races A and B and Materials and Methods publication 6 Apr. 2018. were used in the development of the germ- This research was supported by USDA/AMS plasm C-171, which was released in 1973 and Maintenance of physiological races of P. Award Agreement 12-25-G-0543 and USDA/ was resistant to races A, B, and C (Thomas phaseoli. Isolates of P. phaseoli character- AMS Award Agreement 2-25-B-1059. and Fisher, 1979). In 1975, a fourth race of ized as races E and F were maintained in pure WethankPaulLaytonforhisworktolocateandacquire downy mildew, designated D, was identified. cultures for long-term storage at 20 Cin seed from the wild population of P. polystachios. PI 195342, used since 1972 as a source of screw-cap test tubes containing sterile water Mention of a trademark, proprietary product, or resistance to races A, B, and C, was sus- and corn (Zea mays L.) and/or cucumber vendor does not constitute a guarantee or warranty ceptible to race D (Thomas and Blount, of the product by the authors and does not imply its (Cucumis sativus L.) seeds as substrates. Iso- approval to the exclusion of other products or 1976). A germplasm release designated lates of P. phaseoli races E and F were vendors that may also be suitable. B2C (PI 549515), made jointly by USDA- reactivated by transferring to the semiselective 1Corresponding author. E-mail: luisa.santamaria@ ARS and the Delaware Agricultural Experi- media lima bean agar (Calvert et al., 1960) or oregonstate.edu. ment Station in 1976, had resistance to modified pea agar (Dance et al., 1975). After

HORTSCIENCE VOL. 53(6) JUNE 2018 777 transfer, the cultures were maintained in an 2003 to 2010. Nineteen accessions were from to races E and F of P. phaseoli was started. The incubator set at 20 C. the USDA-National Plant Germplasm Sys- cultivars used in the study were Cypress, Inoculum production and hypocotyl tem and 16 from the International Center for Dover Tucker, Maffei 15, Jackson Wonder, screening of germplasm. Inoculum of P. Tropical Agriculture. The remaining eight and germplasm release B2C. Parents for the phaseoli was produced and maintained on were cultivars obtained from commercial crosses were chosen based on their disease hypocotyls of the susceptible lima bean culti- seed suppliers. A species native to eastern reaction in previous disease screening: ‘Cy- var Concentrated Fordhook. Untreated seeds North America, Phaseolus polystachios (L.) press’ and ‘Dover Tucker’ are resistant to race were rinsed in sterile distilled water to remove BSP, was also screened to determine its E but susceptible to race F, whereas ‘Maffei any surface contaminants and to begin the reaction to races E and F in the field (four 15’ and B2C are susceptible to race E and imbibition process. The seeds were placed in plants for each race), and in a dew chamber resistant to race F (Table 1). ‘Jackson Wonder’ 10-cm plastic pots with a commercial plug and (five plants for each race) during 2010. Seed is susceptible to both races. Crosses were seedling mix, five to six seeds per pot, placed used for screening was collected from a wild made between ‘Jackson Wonder’ and the close together. The pots were placed on heat population in Maryland and subsequently resistant parents in the greenhouse. The F1 mats set to 26 C, on greenhouse benches. submitted to the USDA-NPGS collection, plants from these crosses were planted in the Four days after seeding when hypocotyls accession number W636674. The species P. greenhouseandtheseedfromeachF1 plant began to emerge, some potting mix was gently polystachios, which was not previously iden- was harvested and maintained separately dur- removed by hand from the pots to reveal more tified as a host of P. phaseoli, is closely ing inoculation to facilitate detection of acci- of the developing hypocotyls. Emerging seed- related to lima bean (Delgado-Salinas et al., dental self-pollination. Five F2 populations lings with cotyledons were watered lightly to 1999). (Tables 3–5) were screened to observe the soften seedcoats and whenever possible, seed- Lima bean accessions were evaluated in segregation pattern of the resistance reaction. coats were removed by hand. Inoculum was the field for their reactions to two races of P. A total of 254 F2 plants from ‘Jackson prepared by harvesting 7-d-old infected hypo- phaseoli separately at the University of Del- Wonder’ (susceptible) · ‘Cypress’ (E resis- cotyls and stem material which were cut into aware’s Agricultural Experiment Stations in tant) and 315 F2 plants from ‘Jackson Wonder’ · small (3–6 mm) sections on a sterile surface. Newark (race E) and Georgetown (race F) in ‘Dover Tucker’ (E resistant) were screened. A This material was then mixed with sterile 2004, 2005, 2006, 2009, and 2010. Plots were total of 209 F2 plants from ‘Jackson Wonder’ distilled water to make a slurry. The slurry established during the first week of July in (susceptible) · ‘Maffei 15’ (F resistant) and was placed onto expanding seedlings in direct each year and arranged in a randomized 271 F2 plants from ‘Jackson Wonder’ (suscep- contact with the new tissue. The pots were complete block design with three replica- tible) · B2C (F resistant) were screened. From placed in a dew chamber for up to 7 d. tions. Each plot consisted of 4.6-m row with the E-resistant by E-resistant test cross ‘Dover Symptoms of stunting and distortion typical rows spaced 0.76 m apart. The cultivars Tucker’ · ‘Cypress’, 223 F2 plants were of the disease developed within 6 d with selected were planted by hand or hand- screened. The F2 seeds were planted in 10-cm abundant mycelia and sporangia produced operated vegetable seeder, 12 seeds/m row. pots in seed starter substrate and grown until inside of the dew chamber. The two races of For some cultivars with limited seed, evalu- hypocotyl emergence. P. phaseoli were maintained in separate ation plots were half the length (2.44 m row). At the time of hypocotyl emergence from digital-controlled dew chambers (Percival Susceptible cultivars (Eastland race E and the substrate, the pots were transferred to two Scientific, Inc., Perry, IA). The dew chambers 184-85 race F) were planted every fifth row separate greenhouse humidity chambers with were set at a continuous 20 C, 94% ± 1% rela- as controls and to facilitate pathogen spread. wick mats and timer-controlled misting sys- tive humidity, and continuous 50 mmol·m–2·s–1 Each year, plots were inoculated in early tem as described previously to provide opti- of photosynthetic active radiation. When September at the newly formed pod (pin) to mal conditions for infection. The F2 seedlings exhibited symptoms of downy flat pod stage. Inoculum was prepared as populations were inoculated with either race mildew, the hypocotyls were harvested to previously described and applied with a back- E or F by applying a 10-mL suspension of 1 · prepare inoculum. Lines may be screened pack low-pressure manual pump sprayer 103 sporangia/mL of P. phaseoli and at 5 and for resistance at the seedling emergence (SOLO backpack sprayer, model 425; SOLO 10 d postemergence by spray inoculation to stage or later, during early pod set. When Incorporated, Newport News, VA) in the ensure that there were no escapes from in- screened at seedling emergence, suscepti- early evening when temperatures were be- fection. The plants were visually evaluated ble plants develop symptoms on hypocotyl tween 18 and 21 C. Plants were sprayed until for infection daily over a 7-d period and tissue and resistant plants do not. For runoff with a sporangial suspension contain- infection confirmed by making scrapings of screening at the seedling emergence stage, ing 1 · 103 sporangia/mL of P. phaseoli, and hypocotyls on microscope slides and observ- the inoculum was prepared as previously the inoculation was repeated 7–10 d later. ing the presence or absence of sporangia described. For larger studies, 200gof Beginning the first night after inoculation, the of P. phaseoli. infected tissue and 1000 mL of water plants were misted intermittently each night Evaluation of a population RILs of P. filtered by reverse osmosis were placed in using Micro-Bird misting nozzles (Rain Bird lunatus for resistance of downy mildew a 2-L beaker and mixed thoroughly with Corp., Glendora, CA) from 1800 to 0600 HR incited by P. phaseoli race E in 2011. A set a glass stirring rod; the suspension was re- to increase humidity and leaf wetness dura- of 121 F9 RILs from the cross ‘Bridgeton’ · frigerated at 5 C for 30 min. Finally, plant tion. Irrigation was provided as needed ‘Jackson Wonder’ and the two parental lines tissue pieces were removed by passing the throughout the season using trickle tape in were screened in the field for their reaction to solution through a sieve (U.S. Standard test Newark and overhead irrigation in George- race E of P. phaseoli. Ten seeds of each line sieve No. 18). The concentration of sporangia town. The middle 2.3 m of each row was were planted in the field on 6 July 2011. The was determined from a sample of this suspen- evaluated for susceptibility 10–15 d after the plants were inoculated at the pin pod to flat sion using a hemocytometer (Bright-LineÒ; second inoculation. The plants were rated for pod stage with a sporangial suspension as American Optical Corporation, Buffalo, NY) signs of infection (white fluffy mycelium on previously described. Individual plants were and a compound microscope at ·200. The shoot tips, pins, and flat pods) and symptoms rated as susceptible or resistant on 21 Sept. suspension was adjusted to a density of 1 · 103 of downy mildew (red line surrounding in- 2011 3 months later. Ratings were based on sporangia/mL. fected areas on pods). If necessary, confir- the presence of signs and symptoms of downy Evaluation of P. lunatus accessions and mation of infection was made by evaluating mildew. commercial cultivars in the field for reaction samples morphometrically for sporangia of Statistical analysis. Observed ratios of to downy mildew incited by P. phaseoli races P. phaseoli using a compound microscope. resistant to susceptible plants from resistant · E and F. A total of 43 P. lunatus genotypes Inheritance of resistance of F2 populations susceptible test crosses were compared with (35 accessions and eight cultivars) were of P. lunatus to races E and F of P. phaseoli. the expected ratio of 3 resistant : 1 susceptible screened in the field for their reaction to races To facilitate breeding for resistance to downy for F2 populations and 1 resistant : 1 susceptible E and F of the downy mildew pathogen from mildew, a study of the inheritance of resistance for the RIL population to test for monogenic

778 HORTSCIENCE VOL. 53(6) JUNE 2018 Table 1. Results of screening for downy mildew races E and F resistance in small-seeded lima bean cultivars and germplasm accessions and Phaseolus polystachios germplasm in the field and greenhouse from 2003 to 2010. Reaction to Reaction to Years of field Accession no. Name Germplasm type Growth habit Seed shape Seed color Origin race E race F screening Phaseolus lunatus cultivars obtained from commercial seed suppliers Eastland Select Cultivar Bush Flat Green USA R S 2004, 2005 184-85 Cultivar Bush Flat Green USA R S 2003, 2004 Cypress Cultivar Bush Flat Green USA R S 2003, 2004 C-elite Select Cultivar Bush Flat Green USA R S 2003, 2004 Eastland Cultivar Bush Flat Green USA S R 2003, 2004, 2005 Maffei 15 Cultivar Bush Flat Green USA S R 2003, 2004 8-78 Cultivar Bush Flat Green USA S R 2003, 2004 P. lunatus germplasm obtained from the USDA collection PI 549456 Dover Tucker Cultivar Bush Flat Green USA R S 2004, 2005 PI 189403 Landrace Vine Flat Red Guatemala R S 2004, 2005, 2009 PI 549508 Bridgeton Cultivar Bush Flat Green USA R S 2009 PI 549515 B2C Breeding Bush Flat Green USA S R 2004, 2005 PI 195342 Landrace Vine Flat White Guatemala S R 2009 PI 198092 Baby Potato Cultivar Bush Flat White Morocco S S 2004, 2005 PI 360715 Cultivar Bush Flat Red/black Philippines S S 2004, 2005 PI 549454 Thaxter Cultivar Bush Flat Green USA S S 2004, 2005 PI 549472 Henderson Baby Cultivar Bush Flat White USA S S 2004, 2005 PI 550305 Dixie Butter White 128 Cultivar Bush Round White USA S S 2004, 2005 P. lunatus germplasm obtained from the CIAT collection G25208 Early Thorogreen Cultivar Bush Flat Green USA S S 2004, 2005 G25191 Landrace Bush Flat Pink/black Philippines S S 2004, 2005 G26007 Henderson Cultivar Bush Flat White USA S S 2004, 2005 G26354 Willowleaf Landrace Bush Flat Red USA S S 2004, 2005 G25137 Jackson Wonder Cultivar Bush Flat Buff/purple USA S S 2004, 2005 G25138 Allgreen Cultivar Bush Flat Green USA S S 2004, 2005 G26200 Dwarf White Cultivar Bush Flat White Ghana S S 2004, 2005 G25143 Henderson Bush Cultivar Bush Flat White USA S S 2004, 2005 G25270 De Tupolikanie Landrace Bush Flat White Morocco S S 2004, 2005 G25139 Thaxter Cultivar Bush Flat White USA S S 2004, 2005 G26216 Baby Bush Cultivar Bush Flat White USA S S 2004, 2005 G25142 Early Thorogreen Cultivar Bush Flat Green USA S S 2004, 2005 G26173 YUCATAN 19 Landrace Bush Flat Buff/purple Mexico S S 2004, 2005 G25667A Black and Buff Bush Cultivar Bush Flat Black USA S S 2004, 2005 G26215 Purple Bush Landrace Bush Flat Buff/purple Liberia S S 2004, 2005 G25176 Jackson Wonder Cultivar Bush Flat Buff/purple USA S S 2004, 2005 P. polystachios germplasm maintained in the USDA collection W6 36674 Wild population Vine Very small Brown USA R/S R/S 2010 R = resistant reaction; S = susceptible reaction; CIAT = International Center for Tropical Agriculture. inheritance of resistance using the chi-square Nine large-seeded genotypes were screened populations from the crosses between downy goodness of fit test. Observed ratios for in the field for resistance to downy mildew mildew–susceptible ‘Jackson Wonder’ and resistant · resistant test crosses were com- races E and F (Table 2). The germplasm release the race E–resistant cultivars Cypress and pared with a 15 resistant : 1 susceptible ratio F-169 (PI 549514) was resistant to race E but Dover Tucker segregated in a 3:1 (resistant/ to test for independence of the resistance susceptible to race F and the germplasm release susceptible) ratio with P > 0.05. The c2 loci. MRF-79 (PI 549524) was resistant to race F but values were 2.09 (P = 0.1484) for ‘Jackson susceptible to race E. Two other USDA Ford- Wonder’ · ‘Cypress’ and 1.37 (P = 0.2421) · Results hook releases which had been bred for re- for ‘Jackson Wonder’ ‘Dover Tucker’ sistancetodownymildewracesA,B,C,or (Table 3), indicating that in both cultivars, Evaluation of germplasm for resistance D, ‘F-1072’ and ‘MRF 90-1’, were not resistant a single dominant gene conferred resistance to races E and F of downy mildew. Of the to either race E or F. to race E of P. phaseoli. In the R · R cross · seven baby lima bean cultivars obtained Two of four plants of P. polystachios that ‘Dover Tucker’ ‘Cypress’, all 223 F2 plants from commercial seed suppliers that were were inoculated in the field with race E and were resistant to race E. This result does not screened in inoculated field plots in 2003– three of four plants inoculated with race F fit a 15:1 (resistant/susceptible) ratio to sug- 05, four were resistant to race E and suscep- exhibited signs and symptoms of infection of gest the presence of two independent domi- tible to race F: ‘Eastland Select’, ‘184-85’, P. phaseoli identical to the reaction produced nant resistance genes; the c2 value is 14.94 ‘Cypress’, and ‘C-elite Select’ (Table 1). on lima bean. Samples taken from these (P = 0.0001). The observed result suggests Three of the commercial cultivars were plants were confirmed morphometrically to that these two cultivars carry the same re- resistant to race F and susceptible to race be due to infection by P. phaseoli, indicating sistance gene or two different resistance E: Maffei 15, ‘Eastland’, and ‘8-78’. Of the that P. polystachios is an alternate host for alleles at the same locus (Table 3). 26 small-seeded accessions screened in in- downy mildew. In later dew chamber evalu- Four to 10 individual plants were rated for oculated field plots, three were resistant to ations, five of five plants inoculated at the their reaction to race E for each of the race E and susceptible to race F, ‘Dover hypocotyl stage were susceptible to race E and ‘Bridgeton’ · ‘Jackson Wonder’ RILs and Tucker’ (PI 549456), PI 189403, and two of five plants were susceptible to race F. parent lines. The cultivar Bridgeton is re- ‘Bridgeton’ (PI 549508). Two accessions, These preliminary results suggest that this par- sistant to race E and Jackson Wonder is B2C (PI 549515) and PI 195342, were ticular wild population of P. polystachios has susceptible. All of the plants in each RIL resistant to race F and susceptible to race E. genes conferring resistance to downy mildew. either had signs and symptoms of downy None of the accessions were resistant to both Determining the inheritance of resistance mildew (rated susceptible) or were without races E and F. to races E and F of P. phaseoli. The two F2 signs and symptoms of the disease (rated

HORTSCIENCE VOL. 53(6) JUNE 2018 779 resistant). Sixty-nine of the lines were rated carry a single dominant gene which confers to race E of downy mildew and others with resistant and 52 were susceptible (Table 3), complete resistance to downy mildew race F resistance to race F that are suitable for which fits a 1:1 segregation ratio with a c2 value (Table 4). commercial use in the MAR. Three of the of 2.39 (P = 0.1222), indicating that resistance However, in Table 5, the plants are race E–resistant cultivars, 184-85, Cypress, to race E is conferred by a single gene. categorized as resistant, partially resistant, and C-elite Select, have become the standard When inoculated, the F2 populations from or very susceptible. The segregation ratio is commercial cultivars in the MAR. The race the crosses between ‘Jackson Wonder’ and suggestive of the presence of recessive or F–resistant cultivar Maffei 15 is also cur- the race F–resistant ‘Maffei 15’ and B2C partially dominant genes that confer partial rently used in commercial production. No resulted in plants with three different disease resistance. These partial resistance genes are cultivar has been identified with resistance to reactions. Some plants were completely re- masked in ‘Maffei 15’, B2C, and their fully both races E and F. Therefore, growers sistant, some were very susceptible (i.e., resistant progeny by the single dominant cannot rely on cultivar resistance to manage exhibited obvious mycelial growth and died), gene for resistance, which makes determin- this disease. The University of Delaware lima whereas others were stunted, had some tissue ing the segregation ratio of the genes for bean breeding program is actively working to death, but did not die. Mycelium and sporan- partial resistance in these particular popula- develop baby lima bean cultivars with re- gia of P. phaseoli were detectable on these tions difficult. It is likely that ‘Maffei 15’ and sistance to both races E and F. stunted plants microscopically. In Table 4, B2C carry the same downy mildew resistance Germplasm of the large-seeded Fordhook the plants are categorized as either resistant genes because ‘Maffei 15’ was selected from type from the USDA breeding program was or susceptible, with plants exhibiting partial the germplasm release B2C. identified with resistance to race E (F-169) resistance classified as susceptible. When and race F (MRF-79). These two germplasm classified this way, both populations fit Discussion releases are being used in the University of a 3:1 (resistant/susceptible) ratio with P > Delaware lima bean breeding program to 0.05. The c2 value is 0.10 (P = 0.7490) for In the search for new sources of resistance develop Fordhook cultivars with resistance to ‘Jackson Wonder’ · ‘Maffei 15’ and 1.59 for P. phaseoli in lima bean, screening under races E and F of the downy mildew pathogen. (P = 0.2073) for ‘Jackson Wonder’ · B2C, field and greenhouse conditions helped to Genes for resistance to races E and F are indicating that both ‘Maffei 15’ and B2C identify baby lima bean cultivars with resistance present in cultivars and germplasm releases

Table 2. Results of screening for downy mildew races E and F resistance in large-seeded lima bean cultivars and germplasm accessions in the field and greenhouse from 2004 to 2009. Reaction to Reaction to Years of Accession no. Name Germplasm type Growth habit Seed shape Seed color Origin race E race F field screening Phaseolus lunatus cultivars obtained from commercial seed suppliers MRF 90-1 Breeding Bush Plump Green USA S S 2009 P. lunatus germplasm obtained from the USDA collection PI 549514 F-169 Breeding Bush Plump Green USA R S 2009 PI 549524 MRF-79 Breeding Bush Plump Green USA S R 2009 PI 549464 Fordhook 242 Cultivar Bush Plump White USA S S 2009 PI 549465 Fordhook Bush Cultivar Bush Plump White USA S S 2009 PI 549471 Fordhook Concentrated Cultivar Bush Plump White USA S S 2009 PI 549519 F-1072 Breeding Bush Plump Green USA S S 2009 PI 549479 Concentrated Fordhook Cultivar Bush Plump White USA S S 2009 PI 549518 Dompe 95 Cultivar Bush Flat White USA S S 2004, 2005 R = resistant reaction; S = susceptible reaction.

Table 3. Inheritance of resistance to downy mildew race E in lima bean populations derived from crosses of cultivars Cypress, Dover Tucker, Bridgeton, and Jackson Wonder. Observed plants Expected plants Cross Cross type Test ratio Resistant Susceptible Resistant Susceptible Total c2 value P value Jackson Wonder · Cypress F2 S · R 3: 1 180 74 190 64 254 2.09 0.1484 Jackson Wonder · Dover Tucker F2 S · R 3: 1 227 88 236 79 315 1.37 0.2421 Dover Tucker · Cypress F2 R · R All resistant 223 0 223 Dover Tucker · Cypress F2 R · R 15: 1 223 0 209 14 223 14.94 0.0001 Bridgeton · Jackson Wonder RIL R · S 1: 1 69 52 60.5 60.5 121 2.39 0.1222 R = resistant reaction; S = susceptible reaction.

Table 4. Inheritance of resistance to downy mildew race F in lima bean populations derived from crosses of cultivars Maffei 15, B2C, and Jackson Wonder with genotypes categorized as resistant or susceptible. Observed no. plants Expected no. plants Cross Cross type Test ratio Resistant Susceptible Resistant Susceptible Total c2 value P value

Jackson Wonder · Maffei 15 F2 S · R 3: 1 159 50 157 52 209 0.10 0.7490 Jackson Wonder · B2C F2 S · R 3: 1 212 59 203 68 271 1.59 0.2073 R = resistant reaction; S = susceptible reaction.

Table 5. Inheritance of resistance to downy mildew race F in lima bean populations derived from crosses of cultivars Maffei 15, B2C, and Jackson Wonder with genotypes categorized as resistant, moderately susceptible, or very susceptible. Number of plants Percent of plants Cross Resistant Moderately susceptible Very susceptible Total Resistant Moderately susceptible Very susceptible

Jackson Wonder · Maffei 15 F2 159 21 29 209 76 10 14 Jackson Wonder · B2C F2 212 28 31 271 78 10 11

780 HORTSCIENCE VOL. 53(6) JUNE 2018 produced by the USDA breeders who se- F is conferred by a different single dominant Evans, T.A., R.P. Mulrooney, N. Gregory, and lected for resistance to downy mildew races gene. A partial resistance reaction to race F W.E. Kee. 2007. Lima bean downy mildew: A, B, C, and D. The race E–resistant cultivar was also observed in resistant by susceptible Etiology, and management strategies for Dela- Dover Tucker is reported to be resistant to test crosses and may be conferred by re- ware and the mid-Atlantic Region. Plant Dis. races A, B, and D (Stavely, 1991) and the cessive or partially dominant genes. Only 91:128–135. Evans, T.A., R.P. Mulrooney, and L. Santamaria. race F–resistant germplasm release B2C is race F has been detected in field samples 2005. Development of races of Phytophthora reported to be resistant to races A, B, C, and since 2008 and consequently most of the phaseoli, the causal agent of downy mildew of D (Stavely, 1991). breeding has been focused on the develop- lima bean (Phaseolus lunatus) and the devel- Resistance to races E and F is also present ment of race F–resistant cultivars. The Uni- opment of resistance. Annu. Rpt. Bean Im- in cultivars derived from the USDA’s downy versity of Delaware lima bean breeding provement Coop. 49:15–16. mildew–resistant germplasm releases. The program is currently trialing F-resistant Kee, W.E., J.L. Glancey, Jr., and T.L. Wootten. cultivars Maffei 15 and Eastland are selec- breeding lines in the baby and Fordhook 1997. The lima bean: A vegetable crop for tions from B2C and C-elite Select and were classes to determine whether they have processing. HortTechnology 7:119–128. derived from C-171 (PI 549513), which was commercially acceptable yield, quality, Kirk, P.M., P.F. Cannon, J.C. David, and J.A. a USDA germplasm release with resistance and horticultural characteristics. In addition, Stapler. 2001. Dictionary of the fungi. 9th ed. CABI Publishers, Wallingford, Oxon, to races A, B, and C. The original source of genetic markers for the race F resistance UK. the race E–resistance gene is probably PI gene have been developed and are being Lorz, A.P. 1971. Progress report on ‘‘Lunasta- 189403, which was used in the USDA breed- tested. The partial resistance to race F, chyus’’ amphidiploid derivatives from P. luna- ing program for resistance to races A, B, and which was observed in the test crosses, will tus · P. polystachyus. Annu. Rpt. Bean D. The accession PI 195342, which is also be characterized and mapped with the goal Improvement Coop. 14:46–47. resistant to races A, B, and C, is the probable of using these genes to enhance the durabil- Scuse, M.T. and T. Feurer. 2003. Delaware agri- source of the race F resistance gene. ity of downy mildew resistance in new lima cultural statistics summary for 2001–2002. In these studies, the wild species P. bean cultivars. Delaware Dept. Agr., Dover, DE. polystachios was shown to be an alternate Stavely, J.R. 1991. Lima bean (Phaseolus lunatus Literature Cited L.) development at Beltsville. Annu. Rpt. Bean host of the downy mildew pathogen and the Improvement Coop. 34:155–156. population of P. polystachios studied herein Calvert, O.H., L.F. Williams, and M.D. Whitehead. Thomas, C.A. and V.L. Blount. 1976. Race D of contained individuals with resistance to races 1960. Frozen lima bean agar for culture and Phytophthora phaseoli. Plant Dis. Rptr. E and F. The resistance genes from P. storage of Phytophthora sojae. Phytopathology 60:308. polystachios will not likely be used directly 50:136–137. Thomas, C.A. and V.J. Fisher. 1979. Registration for resistance breeding in lima bean Dance, M.H., F.J. Newhook, and J.S. Cole. 1975. of C-171 lima bean germplasm (reg. no. GP15). because crosses between the two species Bioassay of Phytophthora spp. in soil. Plant Crop Sci. 19:149. do not produce stable diploid progeny Dis. Rptr. 59:523–527. Thomas, C.A. and V.J. Fisher. 1980. Registration (Lorz, 1971). However, additional charac- Delgado-Salinas, A., T. Turley, A. Richman, and of B2C lima bean germplasm (reg. no. GP30). terization of the downy mildew resistance M. Lavin. 1999. Phylogenetic analysis of the Crop Sci. 20:553. cultivated and wild species of Phaseolus Wester, R.E. 1970. A new race of downy genes in this population may help us un- (). Syst. Bot. 24:438–460. mildew on lima beans. Phytopathology derstand the mechanism for resistance in Evans, T.A., C.R. Davidson, J.D. Dominiak, R.P. 60:1856. lima bean. Mulrooney, R.B. Carroll, and S.H. Antonius. Wester, R.E. and R.C. Cetas. 1959. Breeding lima Resistance to race E is conferred by 2002. Two races of Phytophthora phaseoli beans for resistance to downy mildew. Plant a single dominant gene and resistance to race from lima bean in Delaware. Plant Dis. 86:813. Dis. Rptr. Suppl. 257:181–182.

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